Interconnect exhibiting reduced parasitic capacitance variation

ABSTRACT

Adjacent metal lines of an interconnect metallization layer exhibit reduced variation in parasitic capacitance due to the presence of an intervening third metal line. The third metal line is electrically linked to one of the adjacent metal lines and is designed to project into the space between the adjacent metal lines, thereby elevating parasitic capacitance while reducing the range of variation of parasitic capacitance over a known range of critical dimensions. Thickness of the interlayer dielectric formed over the adjacent metal lines can be tailored to trigger penetration of the third metal line within a known range of critical dimensions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a structure and process for reducingvariation in interconnect parasitic capacitance, and in particular, to aprocess and apparatus utilizing insertion of a third metal line betweenadjacent metal lines to reduce interconnect parasitic capacitancevariation.

2. Description of the Related Art

The ever-decreasing feature size of semiconductor devices, and thecorresponding increase in packing density, has rendered integratedcircuits (IC's) more sensitive than ever to signal propagation delays.At this advanced phase of IC development, IC operation is limited by thedelay in propagation of signals between active devices of the circuit,rather than by the speed of the semiconducting devices themselves.

Propagation delay is determined in large part by parasiticresistive-capacitive (RC) delay caused by interconnect linking togethervarious devices of the IC. The magnitude of this RC delay is in turndetermined in large measure by the parasitic capacitance (C_(PAR))component.

In designing IC's, engineers can and do take parasitic capacitance intoaccount. However, this task is complicated by the fact that interconnectparasitic capacitance varies between maximum and minimum values.Therefore, the engineer must ensure that the IC can function over theentire range of variation in interconnect parasitic capacitance.

One important source of variation in interconnect parasitic capacitanceis the variation in critical dimension (CD) of adjacent metal lines ofan interconnect metallization layer. This is illustrated in FIGS. 1A-1C.

FIG. 1A shows a cross-sectional view of an interconnect 100 featuringinterconnect metallization layer 102 including adjacent first and secondmetal lines 102 a and 102 b respectively. Metal lines 102 a and 102 bare positioned over lower interlayer dielectric (ILD) 104. First andsecond metal lines 102 a and 102 b are formed by patterning aphotoresist mask over interconnect metallization layer 102, and thenetching interconnect metallization layer 102 in unmasked areas to stopon underlying lower ILD 104. Next, a second interlayer dielectric 106 isformed over the entire surface, such that dielectric material 106penetrates into inter-line region 108 between metal lines 102 a and 102b.

Parasitic capacitance arising between first metal line 100 a and secondmetal line 100 b obeys the following equation:

C _(PAR)=(∈S)/d,

where

C_(PAR)=parasitic capacitance;

∈=dielectric permittivity;

S=area of the plates of the capacitor; and

d=distance between the adjacent metal lines.

Variation in interconnect parasitic capacitance can be introduced duringfabrication of the interconnect structure. One source of parasiticcapacitance variation occurs during photolithography leading toformation of the metal lines. Specifically, variation in width of thepatterned photoresist mask can in turn induce variation in parasiticcapacitance.

This is illustrated by FIGS. 1B and 1C, which also depictcross-sectional views of adjacent metal lines of an interconnectmetallization layer.

In FIG. 1B, variation in photolithographic processing has led toformation of adjacent metal lines 102 a and 102 b possessing a widthnarrower than that of the adjacent metal lines depicted in FIG. 1A.Because of this changed critical dimension, the distance betweenadjacent metal lines 102 a and 102 b is increased. And, as a directconsequence of Equation (I), the corresponding parasitic capacitance isreduced.

Conversely, FIG. 1C shows a cross-sectional view of adjacent metal linesof an interconnect metallization layer wherein photolithographicprocessing has created metal lines 102 a and 102 b wider than theadjacent metal lines of FIG. 1A. As a result of this changed criticaldimension, the distance between adjacent metal lines 102 a and 102 b isdecreased, and the corresponding parasitic capacitance is increased.

The relation between variation in critical dimension and interconnectparasitic capacitance is shown in FIG. 2. FIG. 2 plots variation incritical dimension (ΔCD) versus parasitic capacitance (C_(PAR)). FIG. 2shows that ΔCD introduces a spectrum of possible parasitic capacitancesinto an interconnect structure. This capacitance variation C_(VAR)ranges between a minimum capacitance (C_(MIN)) wherein ΔCD is a negativevalue (and adjacent metal lines are narrow), and a maximum capacitance(C_(MAX)) wherein ΔCD is a positive value (and adjacent metal lines arewide).

Because variation in parasitic interconnect capacitance governsanticipated signal propagation delay and thereby confines design ofIC's, there is a need in the art for an interconnect structure and aprocess for forming an interconnect structure wherein variation inparasitic interconnect capacitance is minimized.

SUMMARY OF THE INVENTION

The present invention relates to an interconnect structure and a processfor forming an interconnect structure, in which variation in parasiticcapacitance is reduced. This variation reduction is accomplished byinterposing a third metal line between adjacent metal lines of aninterconnect metallization layer. The third metal line is in electricalcommunication with one of the adjacent metal lines. By projecting thethird metal line between the adjacent metal lines, variation inparasitic capacitance is reduced over a range of critical dimensions.

An embodiment of a process for forming an interconnect structure inaccordance with the present invention comprises the steps of forming alower interlayer dielectric over a semiconductor workpiece and formingan interconnect metallization layer over the lower interlayerdielectric. A photoresist mask is patterned over the interconnectmetallization layer, the photoresist including a masked region having acritical dimension and excluding an unmasked region. The interconnectmetallization layer is etched in the unmasked region to leave a firstmetal line separated from a second metal line by an inter-line region, awidth of the first and second metal lines corresponding to the criticaldimension, the first and second metal lines exhibiting a parasiticcapacitance. A conformal middle interlayer dielectric is formed over thefirst and second metal lines and over the lower interlayer dielectric inthe inter-line region. A third metal line is formed over the conformalmiddle interlayer dielectric, the third metal line electronically linkedwith the first metal line and projecting between the first and secondmetal lines in the inter-line region to elevate the parasiticcapacitance and thereby reduce an overall variation in parasiticcapacitance over a range of critical dimensions.

An embodiment of an interconnect structure in accordance the presentinvention comprises a lower interlayer dielectric positioned over asemiconductor workpiece, a first metal line formed over the lowerinterlayer dielectric, and a second metal line formed over the lowerinterlayer dielectric and separated from the first metal line by aninter-line region. A middle interlayer dielectric covers the first andsecond metal lines and the lower interlayer dielectric in the inter-lineregion. A third metal line projects between the first and second metallines and is separated from the first and second metal lines by themiddle interlayer dielectric, the third metal line in electricalcommunication with one of the first and the second metal lines.

The features and advantages of the present invention will be understoodupon consideration of the following detailed description of theinvention and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C show cross-sectional views of various conventionalinterconnect structures featuring adjacent metal lines of differentcritical dimensions.

FIG. 2 plots variation in critical dimension versus parasiticcapacitance for a conventional interconnect structure.

FIGS. 3A-3C show cross-sectional views of interconnect structures inaccordance with various embodiments of the present invention featuringadjacent metal lines of different critical dimensions.

FIG. 3D plots variation in critical dimension versus parasiticcapacitance for an interconnect structure in accordance with the presentinvention.

FIGS. 4A-4C show cross-sectional views of adjacent metal lines in aninterconnect structure in accordance with embodiments of the presentinvention featuring second ILD layers of three different thicknesses.

FIGS. 5A-5C plot simulated variation in critical dimension versusparasitic capacitance for embodiments of interconnect structures inaccordance with the present invention featuring a second ILD layer ofthree different thicknesses.

FIGS. 6A-6E show cross-sectional views of a process flow for forming aninterconnect structure in accordance with one embodiment of the presentinvention.

FIGS. 7A-7F show cross-sectional views of a process flow for forming aninterconnect structure in accordance with an alternative embodiment ofthe present invention.

DETAILED DESCRIPTION

The present invention relates to an interconnect structure and a processfor forming an interconnect structure, wherein a third metal line isdesigned to penetrate between adjacent metal lines of an interconnectmetallization layer. The projecting third metal line is electricallycoupled with one of the adjacent metal lines, and its presence betweenthe adjacent metal lines reduces variation in parasitic capacitance overa range of critical dimensions.

FIGS. 3A-3C show cross-sectional views of adjacent metal lines of threedifferent embodiments of an interconnect structure in accordance withthe present invention. Each interconnect 300 includes adjacent firstmetal line 302 a and second metal line 302 b formed over firstinterlayer dielectric 304. Second interlayer dielectric 306 is formedover first and second metal lines 302 a and 302 b, as well as over firstinterlayer dielectric 304 in inter-line region 308. A secondinterconnect metalization layer 310 is formed over second interlayerdielectric 306 and includes third metal line 310 a. Upper ILD layer 312overlies second interconnect metallization 310.

Where inter-line region 308 is sufficiently large, third metal line 310a penetrates into inter-line region 308 between first metal line 302 aand second metal line 302 b. Third metal line 310 a is connected througha via (not shown) with first metal line 302 a.

As shown in FIGS. 3A-3C, one factor determining the extent of projectionof third metal line 310 a between adjacent metal lines 302 a and 302 bis the critical dimension of metal lines 302 a and 302 b. Adjacent metallines 302 a and 302 b of FIG. 3A possess a moderate critical dimension.FIG. 3B shows that as the critical dimension of lines 302 a and 302 bshrinks, inter-line region 308 between adjacent metal lines 302 a and302 b increases, allowing third metal line 310 a to penetrate intointer-line region 308. Conversely, FIG. 3C shows that as the criticaldimension of metal lines 302 a and 302 b increases, the width of theadjacent metal lines increases, reducing inter-line spacing andexcluding third metal line 310 a.

FIG. 3D plots variation in critical dimension versus parasiticcapacitance for an interconnect structure in accordance with the presentinvention. FIG. 3D shows that the effect of penetration of third metalline 310 a into inter-line region 308 is to reduce the effectivedistance between the plates of the parasitic capacitor formed by firstmetal line 302 a, second metal line 302 b, and intervening second ILD306. This penetration by the third metal line elevates parasiticcapacitance, but more importantly it reduces the variation of parasiticcapacitance over a range of critical dimensions.

FIG. 3D shows that within the range ΔCD′, the adjacent metal lines aresufficiently narrow to permit penetration of third metal line 310 a.This downward projection of third metal line 310 a into inter-lineregion 308 in turn causes an abrupt increase in parasitic capacitance.The effect of this jump is to substantially reduce variation ininterconnect parasitic capacitance (C_(PAR)) over the range of criticaldimensions.

FIGS. 4A-4C illustrate that in addition to the width of the metal lines,another factor determining the extent of projection of the overlyingthird metal line between the adjacent metal lines is the thickness ofthe second interlayer dielectric. FIG. 4A shows an interconnectstructure 300 featuring a second interlayer dielectric 306 of moderatethickness formed over adjacent metal lines 302 a and 302 b. FIG. 4Bshows that where a thin second interlayer dielectric 306 is employed,inter-line spacing 308 between adjacent metal lines 302 a and 302 bincreases. This permits third metal line 310 a to penetrate intointer-line region 308 and thereby reduce variation in parasiticcapacitance. Conversely, FIG. 4C shows that as the thickness of thesecond interlayer dielectric 306 increases, second interlayer dielectric306 excludes third metal line 310 a from inter-line region 308,preventing reduction in parasitic capacitance variation.

Thus, a method of reducing parasitic capacitance variation of aninterconnect in accordance with the present invention requires thatvariation in critical dimension exhibited by a given photolithographicprocesses first be determined. Then, simulating the inter-line distancecreated by metal lines of varying critical dimensions, optimum thicknessof a conformal interlayer dielectric separating the adjacent metal linesfrom the projecting third metal line is determined.

Performance of an interconnect structure in accordance with the presentinvention is optimized where the middle interlayer dielectric possessesa thickness such that the known range of critical dimension induces theoverlying third metal line to project into the inter-line region andthereby reduce variation in parasitic capacitance.

FIGS. 5A-5C plot simulated variation in critical dimension (ΔCD) versusparasitic capacitance (C_(PAR)), for embodiments of an interconnectstructure featuring adjacent metal lines having a critical dimension of0.5 μm separated by an inter-line distance of 0.5 μm, where the secondinterlayer dielectric has thicknesses of 0.15 μm, 0.2 μm, and 0.25 μmx,respectively. In each of FIGS. 5A-5C, variation in critical dimension(ΔCD) of the adjacent metal lines ranges between a negative value (ΔCDleading to narrower metal lines) and a positive value (ΔCD leading tobroader metal lines).

FIG. 5A plots ΔCD vs. C_(PAR) for anohter embodiment of the presentinvention in which the second interlayer dielectric has a thickness of+0.15 μm. FIG. 5A shows that given this ILD thickness, the overlyingthird metal line begins to project between the first and second metallines where ΔCD approaches +0.1 μm. This is evidenced by the abrupt risein parasitic capacitance at that ΔCD.

FIG. 5B plots ΔCD vs. C_(PAR) for another embodiment of the presentinvention, wherein the thickness of the second interlayer dielectric is0.2 μm. With this embodiment, projection of the third metal line betweenthe first and second metal lines, and the corresponding reduction ofparasitic capacitance variation, occurs at a ΔCD about +0.05 μm.

FIG. 5C shows yet another embodiment, wherein thickness of the secondinterlayer dielectric is 0.25 μm. In this embodiment, projection of thethird metal line between the first and second metal lines to boostparasitic capacitance and thereby reduce capacitance variation occurs ata ΔCD approaching zero.

Reduction in parasitic capacitance variation demonstrated in FIGS. 5A-5Cis summarized below in TABLE A.

TABLE A REDUCTION IN CAPACITANCE VARIATION ILD ACTUAL PROJECTED ACTUALTHICKNESS C_(MAX) (F) C_(MIN) (F) C_(MIN) (F) PROJECTED ACTUAL FIG. #(μm) (@ + 1 μm) (@ − 1 μm) (@ − 1 μm) C_(VAR) (F) C_(VAR) (F) 5A 0.15 3.1 × 10⁻¹⁶ 1.8 × 10⁻¹⁶ 2.85 × 10⁻¹⁶  1.3 × 10⁻¹⁶ 0.25 × 10⁻¹⁶ 5B 0.203.15 × 10⁻¹⁶ 1.55 × 10⁻¹⁶  2.45 × 10⁻¹⁶ 1.60 × 10⁻¹⁶ 0.70 × 10⁻¹⁶ 5C0.25 2.95 × 10⁻¹⁶ 2.0 × 10⁻¹⁶ 2.45 × 10⁻¹⁶ 0.95 × 10⁻¹⁶  0.5 × 10⁻¹⁶

As shown by FIGS. 5A-5C, modeling can enable determination of theoptimum thickness of the second ILD layer necessary to achieve smallinterconnect parasitic capacitance over the ΔCD of a givenphotolithography process.

FIGS. 6A-6E show cross-sectional views of a process for forming aninterconnect structure in accordance with one embodiment of the presentinvention. FIG. 6A shows the starting point for the process, whereinfirst interconnect metallization layer 602 is formed over firstinterlayer dielectric 604. First interlayer dielectric 604 overlies asemiconductor workpiece (not shown) in which active semiconductingdevices are fabricated.

FIG. 6B shows patterning of photoresist mask 605 over first interconnectmetallization layer 602. Unmasked portions 606 of first interconnectmetallization layer 604 are then etched to stop on first interlayerdielectric 604, forming first metal line 602 a and adjacent second metalline 602 b. In this manner, photolithographic resolution ultimatelydetermines the width of first and second metal lines 602 a and 602 b.The present invention suppresses variation in parasitic capacitanceattributable to variation in critical dimension exhibited during thisfabrication step.

FIG. 6C shows the next step, wherein the patterned photoresist mask isstripped, and second interlayer dielectric 606 is formed. Secondinterlayer dielectric 606 conforms to the sidewalls and tops of firstmetal line 602 a and second metal line 602 b. Second interlayerdielectric 606 possesses a thickness substantially less (≈20%) than thethickness of first interconnect metallization layer 602. The actualthickness of second interlayer dielectric 606 may be determined bymodeling as described above in conjunction with FIGS. 5A-5C.

FIG. 6D shows formation of second interconnect metallization layer 610over second interlayer dielectric 606. Second interconnect metallizationlayer 610 is the material from which the third metal line projectingbetween first metal line 602 a and second metal line 602 b will beformed.

Second interconnect metallization layer 610 can form the next successivemetallization layer intended to carry the electrical signals of activedevices. However, this is not the preferred form of the invention due tothe relative thinness of underlying second interlayer dielectric 606.Thinness of underlying ILD layer 606 may fail to ensure the integrity ofelectric signals communicated along this interconnect metallizationlayer.

FIG. 6E shows completion of the process, wherein a second photoresistmask (not shown) is patterned over second interconnect metallizationlayer 610, and second interconnect metallization layer 610 is etched inunmasked regions to form third metal line 610 a in electricalcommunication with either first metal line 602 a or second metal line602 b. Third metal line 610 a is positioned above and between first andsecond metal lines 602 a and 602 b. A portion of third metal line 610 aprojects far enough into inter-line region 608 between first metal line602 a and second metal line 602 b to suppress variation in parasiticcapacitance as described above.

The interconnect structure and process flow in accordance with thepresent invention offers a number of important advantages overconventional structures and processes. One key advantage is substantialreduction in interconnect parasitic capacitance variation. Becausevariation in critical dimension of a particular photolithography processcan be determined, and because thickness of the second interlayerdielectric can be carefully controlled, the extent of projection of theoverlying third metal line between the adjacent metal lines can beprecisely calculated. In this manner, the range of parasitic capacitanceexhibited by the interconnect structure is narrowed, granting the ICdesigner greater certainty in anticipating the RC delay experienced bythe active semiconductor devices.

Another advantage of the process in accordance with the presentinvention is its ready integration into existing process flows.Specifically, because the third metal line must merely be linked withone of the adjacent metal lines, the additional interconnectmetallization layer is relatively simple to fabricate. No separate powersupply or ground contacts for this layer are required, and themetallization layer need not contact other signal-carrying interconnectmetallization layers.

Although the invention has been described in connection with onespecific preferred embodiment, it must be understood that this inventionis not limited to this particular embodiment. Various othermodifications and alterations in the structure and process will beapparent to those skilled in the art without departing from the scope ofthe present invention.

For example, the interconnect layers described above can be composed ofa variety of materials. Interconnect metallization can be composed ofany electrically conducting material, with the most widely usedmaterials being aluminum, copper, and tungsten metals. Similarly,interlayer dielectric can be composed of any conforming dielectricmaterial, with the most popular materials being chemical vapor depositedsilicon oxide, borophosphosilicate glass, and tetraethylorthosilicate.

Moreover, while the above figures illustrate implementing this inventionin an interconnect structure formed by direct masking and etching of aninterconnect metallization, the invention is not limited to thisspecific fabrication process.

Copper is increasingly employed as an interconnect metallizationmaterial. However, because of difficulty in selectively etching copperrelative to interlayer dielectric, interconnect structures utilizingcopper are formed by a damascene technique. In damascene processing,interconnect metallization is formed within an ILD trench, and thenmetal is removed outside of the ILD trench by chemical-mechanicalpolishing to form the adjacent interconnect metal lines.

The damascene technique described above suffers from the same variationin critical dimension described extensively above. Specifically, the ILDtrench defining the width of the metal line is generally created byphotolithographic processes.

Therefore, FIGS. 7A-7F show an alternative embodiment of the presentinvention, wherein an interconnect structure featuring the thirdprojecting metal line is formed using a damascene-type process. FIG. 7Ashows the first step of the process, wherein first photoresist mask 705is patterned over a middle interlayer dielectric 704 overlying a lowerinterlayer dielectric 701. Middle interlayer dielectric 704 is thenetched to stop an lower interlayer dielectric 701 in unmasked regions707, forming trenches 709 a and 709 b.

FIG. 7B shows the next step in the process, wherein the firstphotoresist mask is stripped and first interconnect metallization layer702 is formed over the surface. First interconnect metallization layer702 penetrates into trenches 709 a and 709 b during this step.

FIG. 7C shows the next step in the process, wherein first interconnectmetallization 702 is removed outside of trenches 709 a and 709 b bychemical-mechanical polishing. This step leaves first metal line 702 aadjacent to second metal line 702 b separated by middle interlayerdielectric 704 in inter-line region 708.

FIG. 7D shows the next step, wherein middle interlayer dielectric 704 isremoved in inter-line region 708, followed by formation of thin,conformal interlayer dielectric 706 over the entire surface. Thininterlayer dielectric 706 conforms to the exposed sidewalls of adjacentmetal lines 702 a and 702 b, and penetrates into inter-line region 708.

FIG. 7E shows the next step in the alternative process flow, whereinsecond interconnect metallization layer 710 is formed over thin,conformal ILD layer 706. Depending upon such factors as the thickness ofthin ILD layer 706 relative to the width of inter-line region 708 and ofmetal lines 702 a and 702 b, second interconnect metallization layer 710penetrates to a varying degree into inter-line region 708.

FIG. 7F shows completion of the interconnect structure, wherein secondinterconnect metallization layer 710 and conformal interlayer dielectric706 are removed outside of inter-line region 708 by chemical-mechanicalpolishing. This step leaves third metal line 710 a projecting betweenadjacent metal lines 702 a and 702 b. A passivation or succeeding ILDlayer 712 is then formed over the interconnect structure.

Given the above description and the variety of embodiments disclosedtherein, it is intended that the following claims define the scope ofthe present invention, and that processes within the scope of theseclaims and their equivalents be covered hereby.

What is claimed is:
 1. An interconnect structure comprising: a lowerinterlayer dielectric positioned over a semiconductor workpiece; a firstmetal line formed over the lower interlayer dielectric; a second metalline formed over the lower interlayer dielectric and separated from thefirst metal line by an inter-line region; a middle interlayer dielectriccovering the first and second metal lines and the lower interlayerdielectric in the inter-line region; and a third metal line projectingbetween the first and second metal lines and separated from the firstand second metal lines by the middle interlayer dielectric, and whereineach of the first, second and third metal lines has a thickness, andwherein a thickness of the middle interlayer dielectric is approximately20% that of said metal line thickness.
 2. An interconnect according toclaim 1 wherein the first, second and third metal lines comprisesubstantially copper.
 3. An interconnect according to claim 1 whereinthe first, second and third metal lines comprise substantially aluminum.4. An interconnect according to claim 1 wherein the middle interlayerdielectric is composed of a dielectric material selected from the groupconsisting of chemical vapor deposited oxide, borophosphosilicate glass,and tetraethylorthosilicate.